Conventionally, the process of a fluid substance discharged from a reservoir is controlled by a valve for providing the fluid substance at the specific time, and it is widely used in the industry. For example, a dust collector with a bag filter takes advantage of high pressure air discharged from a reservoir to clean the dust on a filter and then collect the dust in the bag. Most of valves applied in an automatic control system for driving an operating unit or in a reservoir for introducing the required air into every operating unit are electromagnetic valves.
As we know, the pressure and the temperature in the reservoir will change during the discharged process, and the properties of the fluid substance in the reservoir will also vary all the time. So, it is hard to measure the accurate flow rate of the fluid substance discharged from the reservoir.
In the commonly used flow meter in the industry, such as the orifice meter or the venturi meter, there is a difference in pressure between the upstream and downstream regions of the valve. In other words, these kinds of flow meters are just suitable for the fluid substance in a steady state, and the flow rate of the fluid substance is usually indicated by pressure differences between the upstream and downstream regions of the flow meter.
For convenience, the Instrument Society of American proposed a measurement for determining the flow rate (American National Standard). Referring to FIG. 1, a reservoir 11 is connected with a throttle 12, a thermocouple 13, and a flow meter 14 in series. Two pressure taps 15,16 are provided at the upstream and downstream regions of the valve 17 in the testing area 18. The method for calculating the flow rate is to determine the difference in pressure at the pressure taps 15,16. This equipment needs a great amount of the fluid substance to maintain a steady state during the discharged process, but it is hard for a factory to set up such a flow meter with a huge volume of the fluid substance. More importantly, the condition of a valve used in the discharged process, e.g. an electromagnetic valve, is usually in a dynamic state, rather than in a steady state (unless the pressure is low). However, it is not reasonable the flow rate of the fluid substance in a steady state is used to represent all the conditions of the fluid substance (including the dynamic state). So, it is tried by the applicant to deal with this problem encountered by the prior art.